Such probes may be, for example, pH measuring probes for measuring the pH of a liquid or food, such as meat, and they may be installed in portable measuring devices. In the simplest embodiment, such measuring probes have two electrodes in a housing. A chamber holding a second electrode formed by a polymer protolyte gel, for example, is usually provided between the first electrode and the housing.
In manufacturing such measuring probes, it is important for the inner electrode to have a high resistance in comparison with the outer electrode, and the amount of measuring liquid exchange between the liquid in the first electrode and the liquid in the second electrode should be minimized. Essentially two different designs are known for such polymer electrolyte measuring probes. First, there are measuring probes made completely of glass, and second, there are also measuring probes made of plastic but with the inner electrode situated in a glass tube. The design and functioning of this electrode are explained briefly below.
In the manufacture of glass electrodes, two electrode chambers are first produced by glass blowing, with the inner electrode being sealed by a pH glass diaphragm and the outer electrode being situated in an electrode chamber which is fused onto the closed end of the inner electrode. This results in a kind of double-walled glass beaker design.
A liquid electrolyte is cast into the chamber of the inner electrode. The inner chamber is sealed using a foam cylinder which seals the inner chamber like a plug, and a silver wire is passed through the foam cylinder until reaching the electrode bottom. To further seal the inner chamber, silicone is extruded into the rear area of the glass tube. Measuring probes preassembled in this way must then cure for a couple of hours to secure the silver wire in the inner chamber.
The silver wire of the inner electrode is then connected to a coaxial line; for shielding reasons, it is important for the soldered end of the silver wire together with the inner insulation of the coaxial line to be immersed in the glass tube of the inner electrode. Because of the small diameter of the glass tube, soldering cannot be performed inside the glass tube, so a coil in the form of a mechanical spring or spiral must typically be coiled at the end of the silver wire protruding out of the glass tube. After soldering, this coil, which forms the end of the silver wire, is compressed by the insulation on insertion of insulation.
The chamber of the outer electrode is closed using foam and sealed by a silicone material in the same way. A plastic cap is pushed onto the end of the probe for tensile strain relief of the electrode rod and the cap is cast there using an adhesive, typically a two-component epoxy resin. A polymer electrolyte is added to the outer chamber of the measuring probe under a vacuum.
Such glass electrodes are extremely expensive to manufacture because of the multitude of different manufacturing steps. Another problem is that, because of the material of the glass electrode and the small amount of space available, a great many complicated manufacturing steps are necessary during assembly of the glass electrode, hardly allowing adequate yield in automated production. The use of glass tubes in particular is problematical here because the various components are frequently assembled inside the fragile glass electrodes or at least in their immediate proximity. In most cases, this prevents the use of manufacturing machines for automation of the manufacturing process. However, at the same time, this also means that because of the plurality of different manufacturing steps and the need for performing them manually for the most part, the corresponding glass measuring probes are very expensive to manufacture.
Meanwhile, there is thus a demand for providing measuring probes for measuring instruments which are simpler and thus less expensive to manufacture without restricting their functionality.
Plastic measuring probes are far more easily manufactured than the measuring probes made of glass as described above. The design of one such plastic measuring probe is described in DE 100 04 583 C2, for example.
The manufacturing steps required for producing such plastic probes are essentially the same as those for producing a measuring probe of glass. Some of the manufacturing steps may be simplified by automation because first of all, the glass sheathing of the electrode need no longer be produced by glass blowing, which is very expensive. Nevertheless it is also necessary here to perform a number of manufacturing steps which have the unwanted effect of making the plastic measuring probe more expensive.
As already described in DE 100 04 583 C2, plastic probes are much sturdier than glass probes, but they are very sensitive to impact, in particular in the axial direction. In addition, it is necessary from an economic standpoint in particular to occasionally refill or replace the electrolyte liquid inside the measuring probe. However, in the case of measuring probes made of plastic, this is possible only to an unsatisfactory extent or not at all. In addition, glass measuring probes are characterized in comparison with plastic measuring probes in that they may be used even when high hygienic demands must be met or when the medium to be measured has a very high temperature, for example. In some cases, glass probes are much better than plastic measuring probes because of the low outgassing of impurities and because of their high thermal stability.
Accordingly, it is desirable to manufacture high-quality measuring probes without using gluing or casting methods, if possible. Further, it is desirable to provide the simplest possible method for manufacturing high-quality probes that is suitable for automation. Yet further, it is desirable to create a method for manufacturing a measuring probe which makes it possible to open the measuring probe again after being manufactured.